Dissolved oxygen content of the water of fish ponds is one of the most important
parameters of water quality, as the oxygen is a vital condition for all the
organisms living in the water and having an aerobic type of respiration. The
solubility of oxygen is influenced by several factors (e.g. air pressure, hydrostatical
pressure, salt content), but in pond fish farms it is generally enough to consider
only water temperature.

Table 1 shows values of oxygen saturation of water as a function of water temperature. In pond fish farming the dissolved oxygen content of the water can be expressed as the absolute quantity of oxygen in one unit of water (mg/l; ppm) or as relative oxygen saturation (%).

Diurnal changes of the dissolved oxygen content of pond water during a period of time t¢¢ - t¢ can be expressed by the following equation:

O2¢¢ - O2¢ = P - R - Y ± A

where

P = quantity of oxygen produced during the photosynthetic process of the phytoplankton, the phytobenthos and the water plants,

R = quantity of oxygen consumed during the respiration of bacterial-, phyto- and zooplankton and organisms of animals and plants,

Y = quantity of oxygen fixed by the mud of the pond bottom,

A = quantity of oxygen dissolved from the atmosphere or in case of supersaturation released to the atmosphere.

Since the most significant positive value (P) of the above formula is dependent on light conditions (the photosynthesis can take place only by utilization of solar energy), the dissolved oxygen content of the fish pond varies over a period of 24 hours.

Figure 1 shows a typical daily change of oxygen concentration of a fish pond. The photosynthesis starts gradually from the early morning hours while the dissolved oxygen content of the water is increasing. The maximum value, in several cases much higher than the saturation level, can be observed early in the afternoon.

From the late afternoon hours photosynthesis is rapidly decreasing and is stopped after dark. The dissolved oxygen content of the water is also decreasing during this period, the minimum value occurring during the summer months.

Those organisms which live in the water of a fish pond and have chlorophyll, produce carbohydrates from the carbon dioxide and the water in the process of photosynthesis by utilization of solar energy, while oxygen is produced as a by-product of the process. In deeper water ponds the biggest part of the oxygen produced comes from the phytoplankton, but in shallow ponds the macrovegetation, the algae and the benthic algae have a dominating role. Among the factors influencing production of fish ponds water temperature and illumination are the most important.

Table 1 The oxygen saturation concentrations in the water as a function of water temperature at 760 torr

Figure 2. Relationship between the oxygen production and illumination in fertilized fish pond

The relationship between illumination and oxygen production can be seen in Figure 2. The intensity of light affects significantly the oxygen budget of pond water. If the intensity of solar radiation is high during a day without overcast, a quantity of dissolved oxygen is produced. This covers the oxygen demand of respiration during the night. In the cases when solar radiation is less intensive because of cloud cover, the oxygen production and thus the oxygen reserve for the night is less. During the night the respiration of organisms in the water does not decrease - or at least not in the same rate - and the oxygen reserve of the pond water can be exhausted. Thus the concentration of dissolved oxygen can decrease below the lethal level for fish.

This situation can be extremely dangerous at the end of a growing season in August, when the length of the days is shorter (Figure 3), the quantity of daytime production is less, and at the same time the biomass is the highest.

Studying the values of oxygen saturation at 2 p.m. and 5 a.m. throughout the growing season, it has been found that in the case when the dissolved oxygen content at 2 p.m. did not reach 10-12 mg/l, the oxygen content decreased to a dangerous level during the hours of sunrise, and depending on the length of the oxygen shortage period, it caused fish kills (Figure 4).

Transparency of the water can be decreased by over-production of phytoplankton too. The over-produced mass of phytoplankton has a self-screening effect and it can be a significant factor in intensively manured and foddered ponds. In these ponds production is limited only to the upper water layer and the production here can not cover the oxygen demand of the lower water layer where respiration is dominant.

If the light conditions are not favourable a very dangerous situation can develop (short day, cloudy sky).

After over-production of phytoplankton, production can be decreased not only because of the self-screening effect but by the disruption of nutrient equilibrium. In critical cases heavy losses of plankton can take place.

The relationship between oxygen production and water temperature in an intensive carp production pond is shown on Figure 5. Although the oxygen production of pond water will be higher and higher due to the increase of temperature, a critical situation occurs in the case of high water temperatures because the rate of oxygen consumption exceeds the rate of production.

Oxygen is consumed in pond water by decomposition of organic materials in the respiration of macro-vegetation, phyto- and zooplankton and bacteria. On the basis of measurements carried out in large scale fish ponds of Hungary it was found that oxygen consumption of the water in a pond full of weeds was nearly completely the result of respiratory action of macro-vegetation.

In another case in a manured but empty fishpond the mass of phytoplankton decreased nearly to zero due to over-production of zooplankton. In this case the zooplankton was the main consumer of oxygen. In the case of a stocked fish pond on the other hand, the mass of zooplankton decreased to such a minimum level due to the consumption by fish that even the chemical destruction of the remaining zooplankton did not make any demonstrable change in oxygen consumption of the pond water.

Fish pond water is a complicated biological complex, thus its oxygen consumption is different from pond to pond under different conditions.

While oxygen production is dependent mainly on light conditions, respiration is mainly determined by water temperature and dissolved oxygen content of the water.

In an intensive channel catfish production pond a correlation has been found between oxygen consumption of pond water and water temperature as shown in Figure 6.

Determination of primary production and of oxygen consumption of pond water has been carried out by the Fish Culture Research Institute. Based on a mathematical analysis of daily oxygen curves, it was found that the intensity of respiration in the fish ponds studied has shown a significant daily rhythm, with an afternoon maximum, and that it has been in direct proportion to oxygen concentration (Figure 7).

Figure 3. Sunrise and sunset at Budapest according to Central European Standard Time

Figure 4. The oxygen concentration in the surface of a fish pond at 0500 and 1400 hours

Figure 5. The oxygen production in intensive fish pond as a function of water temperature

Figure 6. The oxygen consumption of the water of a fish pond as a function of water temperature

Figure 7. Relationship between the oxygen concentration and respiration

Oxygen consumption by the pond bottom is dependent on the oxygen requirement for decomposition of organic material accumulated on the pond bottom and for vital functions of benthos. Oxygen fixation taking place in the water mud interfacial area can also not be neglected. The dissolved oxygen content of the water layer directly in contact with the mud is much lower than that of the upper layers of water.

According to some studies the 24 hour-long oxygen consumption of the pond bottom is 1-3 grams per m2. Calculating with 1 m water depth, the 24 hour-long oxygen consumption of the pond bottom changes between 1 and 3 mg/l.

The different fish species have become adapted to different living conditions during their evolution. Generally all those fish species which live in fast flowing and oxygen rich streams (e.g. Salmonids) need high quantities of oxygen, and only a small decrease of dissolved oxygen can cause losses. At the same time the species which became accustomed to slow motion of the water or stagnant water (e.g. several Cyprinids) need less oxygen and are able to tolerate short periods of oxygen deficiency.

Some fish species are also known, mainly in the tropics, to utilize the oxygen of the atmosphere by cutaneous respiration, intestinal and swim-bladder respiration or by a respiratory organ similar to lungs.

Oxygen consumption by fish is generally given in mg/body weight and kg/hour. Standard oxygen consumption means the quantity of oxygen consumed by the fish without swimming and feeding (quantity of oxygen necessary for subsistence). Standard oxygen consumption is not dependent on oxygen saturation of the water but it is significantly influenced by water temperature.

Figure 8 shows standard oxygen consumption of some fish species as a function of water temperature on the basis of the data of several authors. In the practice of fish culture the active oxygen consumption (the oxygen consumed by swimming and feeding fish) can be used.

Active oxygen consumption by fish has been analyzed very rarely till now because of the technical difficulties of measuring, and available data can not be compared because of inaccurate or imperfect description of the circumstances of measurement.

Table 2, showing oxygen consumption of different fish species, gives the size of fish and an idea of the motion where they have been cited. From Table 3 it is obvious that oxygen consumption by fish can significantly change at a given water temperature depending on the activity of fish. Table 2 shows that in identical conditions oxygen consumption by smaller size fish is higher.

Active oxygen consumption by fish has a strict connection with oxygen saturation of the water. Figure 10 shows the correlation between oxygen saturation and oxygen consumption by fish, for seven fish species. Some scientists are of the opinion that fish are able to sense a decrease of dissolved oxygen content of the water before it causes troubles in their respiration and therefore they look for water of higher oxygen content. If this is not successful and the dissolved oxygen content of the water decreases further, the respiratory motion of the fish is increased as long as the oxygen supply to respiratory muscles permits. This reaches a limit when further acceleration is hindered by lack of oxygen. Fish are tolerant till this level. If the dissolved oxygen content of the water decreases further, the pulsation of the heart slows down and the fish dies after a time depending on the resistance characteristic of the species. Lethal oxygen concentration is the dissolved oxygen content that can not be tolerated by the given fish species for a given period of time.

Table 3 The oxygen consumption (ing/kg/hour) of 100 g rainbow trout (Salmo gairdneri) as a function of water temperature and fish activity (the dissolved oxygen concentration in the water is 80-90%)

Water temperature (°C)

5

10

15

20

25

Reduced metabolism

20

22

72

90

138

Natural conditions without stress effects

100

180

250

280

-

Active metabolism (intensive feeding)

-

-

472

360

-

Forced swimming

-

480

580

544

478

Figure 11 shows that there were no significant losses of fish in an intensively manured fish pond with 0.3 mg/l dissolved oxygen content if the period of oxygen shortage was brief. But more than ten hours of oxygen shortage caused serious losses of fish.

On the basis of investigations on intensive pond culture of common carp in Central Europe, the probability of oxygen deficiency (less than 1 mg/l dissolved oxygen) is nearly continuous from the beginning of July till the beginning of September, but losses caused by oxygen deficiency occur generally in August. This can be explained by the fact that in July the period of oxygen deficiency is only 1-2 hours but in August sometimes twelve hours of oxygen deficiency can occur.

In the practice of pond fish farming oxygen deficiency has been regarded as dangerous mainly because of mass losses of fish. But recent investigations have shown that decreased oxygen saturation can have serious effects on the economy of a fish farm as well.

Increase of toxic effect of different toxic materials can not be neglected in water bodies with low oxygen supply. The low oxygen content disadvantageously influences both the food intake and the utilization of food. Investigations have shown that lower than 25% oxygen saturation occurring before sunrise has a disadvantageous effect on fish-growing.

Keeping the dissolved oxygen content of the pond water nearly at the saturation level makes it possible not only to avoid mass losses of fish but ensures better conversion rates and higher yields in intensive culture.

Diffusion is caused by partial differential pressure of oxygen in the air and in the water.

As the oxygen content of the air is considered to be constant, the rate of diffusion is determined by oxygen saturation of the water layer interfacing with the air. In the case of stagnant water the uppermost layer of water becomes saturated quickly and the convection current of oxygen into the water slows down. The mass transfer rate which determines the rate of diffusion (g/O2/m2/hour) varies in a wide range. It can be seen from a collection of the data of several authors that the mass transfer rate has varied between 0.03-5.0 g/m2/hour depending on the circumstances (Table 4).

In fish ponds the rate of diffusion is dependent on the mixing of water layers caused mainly by wind action. The oxygen intake from the atmosphere by diffusion is 1.5 g/m2/day in small ponds and 4.8 g/m2/day in big ponds, where the wind action is stronger.

Depending on oxygen saturation of the water, diffusion can show a reversed direction. In case of over-saturation of the water oxygen diffuses to the atmosphere. Diffusion is assisted by the mixing of water caused by wind action in this case.

It is well known that wind action has a good effect on the water quality of fish ponds through increased diffusion. In the case of construction of new fish ponds the prevailing wind direction has to be taken into account in order to utilize natural diffusion.

In intensive fish ponds where the oxygen budget of the pond water can be regulated artificially, diffusion by wind action has less importance.

In the case of intensively manured and fed fish ponds, where a significant part of the feed of fish is produced in the water of the fish pond itself, increase of flow rate can not be applied for oxygen saturation.

Artificial aeration means increasing the oxygen content of all or part of the pond water to limits which ensure the oxygen supply to the fish without limiting the production at a given management level.

In extensive fish culture, aeration of fish ponds has a "life-saving" role only in case of oxygen deficiency. With the increasing of the intensity of pond fish culture, because of the accelerated biological activity (intensive manuring, feeding, high stocking density) the natural oxygen supply becomes more and more insufficient and will be a limiting factor in production. Intensive aeration of fish ponds has not only a life-saving role, but it becomes one of the basic factors of production, and ensuring optimal oxygen supply makes possible the maximal utilization of the given biological possibilities.

As already mentioned, food intake of fish is influenced disadvantageously by the decrease of dissolved oxygen content of the water below 20-30 percent. In intensive fish culture the proper oxygen supply has an important role in the economy of production. At the same time saturation values much higher than 100 percent are disadvantageous for the normal vital processes of fish, as they are forced to an increased respiration. Consequently the aeration of fish ponds means the regulation of dissolved oxygen content of the pond water and not necessarily the increase of it.

During the planning of fish pond aeration, one can count on extended water surface areas as potential air intake areas.

Oxygen over-saturation of the surface water layer can be decreased by mixing of the water, and the oxygen can be uniformly distributed in the total water body of the pond. Mixing of the water in shallow ponds such as fishponds has no influence on oxygen production of plankton. Mixing the water during daytime can ensure that significant quantities of oxygen are reserved for the night hours when consumption is predominant.

In pond fish farming the oxygen content of the atmosphere is utilized for enrichment
of pond water with oxygen. Basically there are two methods for oxygen enrichment.
In the first one energy is transferred to the surrounding air by some kind of
air compressor and the air penetrates into the water where it transmits a part
of its oxygen content to the water. In case of the second one energy is transferred
directly to the pond water or other water source and the water having a higher
energy content will be able to take up air from the atmosphere and dissolve
part of its oxygen content. The principal schemes of these aeration processes
are shown in Figure 12.

A water pump is the basic device for the hydraulic type of aeration with which the energy content of the water can be increased. The air intake device can be a cascade, a sprinkler, an ejector or an air intake head. These are generally connected to the pump through a pipeline. In the group of hydraulic type aeration devices, the surface aerators are shown in a separate group in the figure, because of their special construction. Surface aeration can be provided by a simple open impeller or by a centrifugal pump without housing, and its construction is designed specially for pond aeration. The basic unit of air diffusion type aeration devices is the air compressor which can be a root-type blower, a ventilator, a compressor or a membrane pump. The air penetrates the water through porous material such as a perforated tube placed in the pond.

In case of aeration with sprinklers the water jet comes out from the nozzle with high velocity and falls into the water in drops, thus contacting the air on a large surface and dissolving oxygen from it. When the water drops hit the water surface of the fish pond turbulence occurs increasing the diffusion of air into the water. The water jet can be directed down into the water surface forcing air bubbles into the water.

In aeration with an ejector the water is passed through a venturi-type diffusor where its pressure decreases below the atmospheric pressure thus air can penetrate into the water.

Specially designed air intake heads can also be used for aeration. These usually involve a propeller inside a pipe through which water is passing. The propeller is driven by the water current and on the surface of the blade the pressure decreases and an air sucking effect takes place.

Cascades can also be used for aeration, in which the water is broken into small drops, increasing the interfacial area.

Hydraulic type aerators can be applied usefully when water is pumped through a pipeline but they are particularly useful when a natural head is available.

Surface aerators are placed directly in the fish pond and due to the rotation of a paddle type device, either with vertical or horizontal axis, the water is discharged into the air. These aerators generate a vertical or horizontal water current in the fish pond that has an advantageous effect on the oxygen budget.

During air diffusion the air is supplied by various types of compressors or blowers into the water where it is diffused through a variety of diffusors. When the air is diffused through a perforated pipe large bubbles of up to 10 mm in diameter are formed and when the diffuser is a porous material fine bubbles of 2-5 mm in diameter are produced. When the bubbles emerge and pass up to the water surface a part of their oxygen content is dissolved in the water, and also a secondary upwards water movement is generated, creating a mixing effect.

Intensive fish culture systems are generally established in areas where there
is a large quantity and good quality of flow-through water. In such systems
the oxygen requirement of fish is ensured by the inflow water. This means the
fish load capacity of these systems is dependent on the rate of water flow.
The water flow can not be increased beyond a certain limit because of technical,
economic and physiological reasons, although the other elements of the system
would make possible a higher stocking density. The fish load can be increased
to a certain extent by direct aeration of the water, but there are technical
and economic limitations on individual aeration of fish tanks.

By application of pure oxygen, the oxygen supply no longer limits the fish load of the system. Furthermore the oxygen supply can be regulated and the safety of the operation increases. The significant advantage of application of pure oxygen is the high mass transfer rate between the pure oxygen and the water, because of high differences in concentration. In application one has to take into account the costs of oxygen and the investment in devices to dispense it. A well-designed and well-operated oxygen supply system can compensate for the higher investment costs because of its low energy requirement.

Pure oxygen is available in gas or liquid form. For the supply of intensive fish culture systems usually liquid oxygen is used, produced by cryogenic separation. The liquid oxygen of 99 percent purity is stored in tanks with vacuum insulation and evaporated before utilization.

Recently "oxygen generators" have been designed which can produce oxygen of 90 percent purity, removing the nitrogen from the air by a molecule filter.

In order to minimize oxygen consumption devices are needed which ensure the most perfect absorption of oxygen. The operating principle of these is the same as that of aeration devices, but in order to increase the contact time between the gas and the water some technical modifications are needed.

One is bubbling oxygenation where the oxygen gas comes in contact with the water by breaking into bubbles. The rate of oxygen dilution depends mainly on the depth of the water layer, the length of travel of bubbles in the water body and the rate of oxygen feeding. Higher efficiency can be achieved by decreasing the bubble size, thus the contact time and the contact surface increases. But the decrease of bubble size needs a significant quantity of extra energy, and coagulation of bubbles also can happen. The efficiency of oxygen bubbling can be increased by counterflow of oxygen and water (Figure 13/a). Undissolved oxygen can be collected and circulated back to the oxygen supply system (Figure 13/b), or a closed system can be constructed in which oxygen and water are continuously mixed (Figure 13/c).

Where oxygen is injected into the water flowing in a tube at a given point, the efficiency of oxygen dissolving depends on the time the oxygen stays in the pipeline and on the pressure in the tube. Oxygen can be fed into the pipeline through a Venturi tube, as shown in Figure 13/d.

U-tube oxygenation is shown in Figure 13/e, in which bubbles of oxygen are carried along the water current. Thus the contact time and efficiency of oxygen dilution depends on the difference of inflow and outflow water levels and on depth of the "U" tube. This process seems to be very effective in fish culture practice because of its high efficiency, low energy requirement and simplicity.

The above mentioned basic dissolving principles can be found simultaneously in the same device as shown in Figure 13/f.

The equilibrium concentration is the maximum concentration of oxygen which can be dissolved in the water relative to the concentration of oxygen in the gas under the prevailing conditions of temperature and pressure. In fish pond conditions the gas means the atmospheric air, although pure oxygen also gets into the fish pond water by the oxygen production of phytoplankton and other water plants. In some intensive fish farming systems pure oxygen is applied in order to meet the oxygen requirement of the fish.

The equilibrium concentration in gas-liquid systems is expressed by Henry's law as follows:

The partial pressure of one component of a gas mixture is proportional to the volume fraction of that component.

where

P = the pressure of gas mixture (Pa)

Figure 13. a) Counterflow column

Figure 13. b) Gas recycling

Figure 13. c) Enclosed Operation

Figure 13. d)Venturi oxygenator

Figure 13. e) U tube oxygenator

Figure 13.f) Complex system

21 percent of the atmospheric air is oxygen thus the volume fraction is:

The oxygen concentration expressed in mole fraction can be converted into weight fraction as follows:1/

1/ In the following equations the notations k-1 and m-1 are used to denote per kg or per m3

where

thus

The weight fraction can be converted into weight concentration (Cs) as follows:

(kg . m-3)

where

Cs = weight concentration of oxygen at saturation (kg . m-3)

The density depends on temperature as shown in Table 5.

Based on the equations given above, the weight concentration of oxygen at saturation (Cs) can be calculated as follows:

(kg . m-3)

(g . m-3)

Table 5 Density of pure water

Temperature°C

Densitykg/m

Temperature°C

Densitykg/m3

0

999.87

16

998.97

1

999.93

17

998.80

2

999.97

18

998.62

3

999.99

19

998.43

4

1 000.00

20

998.23

5

999.99

21

998.02

6

999.97

22

997.80

7

999.93

23

997.57

8

999.88

24

997.33

9

999.81

25

997.07

10

999.73

26

996.81

11

999.63

27

996.54

12

999.52

28

996.26

13

999.40

29

995.97

14

999.27

30

995.68

15

999.13

When calculating the pressure (p), the atmospheric pressure and the water head have to be taken into account (1 mm H20 = 9.80665 Pa). In the case of a surface aerator, one has to calculate only with the atmospheric pressure. The Cs values at normal atmospheric pressure are shown in Figure 1.

In those places where the elevation is different from sea level, the atmospheric pressure can be calculated with the "barometric level formula" as follows:

q = temperature (°C)b = constant with a value between 1.016 and 1.047In sewage treatment with bubbling aeration usually b = 1.02 is used during the calculations

Generally:

The organic and inorganic materials in the water have an influence on KLa.

K¢La = a KLa

where

K¢La = the modified mass transfer rate of an impure watera = constant, its value lies between 0.7 and 0.9 when the water is biologically treated and 0.5 when the water is mechanically treated.

The Oxygenation Capacity (OC) expresses how many grams of oxygen can be dissolved in 1 m3 of water during one hour by the aerator investigated, at normal atmospheric pressure when the water temperature is 10°C and its initial dissolved oxygen content is zero.

The relation between the Oxygenation Capacity and modified mass transfer rate is shown as follows:

OC = KLa (10°C) . Cs (10°C)

The value of OC at different temperature and atmospheric pressure can be computed with the equation:

where

When the same aerator is used for the oxygenation of a larger water volume than 1 m3 less oxygen can be dissolved. This is why total oxygen intake (Ot) is introduced.

When fine and medium size bubbles are produced, the diameter of the bubbles is larger than the size of the hole through which the air enters into the water . When a large bubble is produced its diameter is smaller than the hole size.

The elevation velocity of the bubbles in ease of different bubble size is shown as follows:

dB < 0,15 mm

VB = 478 500 dB2 (ms-1)

0,15 mm < dB- < 2.10 mm

VB = 758 dB1.25 (ms-1)

2.10 mm < dB < 7.20 mm

VB = 0.0164 dB0.5 (ms-1)

dB > 7.20 mm

VB = 2.24 dB0.5 (ms-1)

In bubble aeration the following ratio is used as a reference to the efficiency of oxygen dissolving:

The value of this ratio in different systems is as follows:

a) fine bubble aeration:

9-10 percent

b) medium bubble aeration:

5 - 6 percent

c) coarse bubble aeration:

3.5- 5 percent

where

Q = air volume (m3h-1)S = density of air (kgm-3)OCV = (g h-1)

The manufacturers of different aerators use characteristic curves that can be expressed as follows:

or

where

S = density of air (kg m-3)w = mole fraction of O2 in air (kg kg-1)

Generally the air volume (Q) is related to normal atmospheric air. In this case:

S = 1.293 kg m-3

and

S . w = 1.293 . 0.232 = 0.3 kg m-3

The equations above are related to a given water depth. The different parameters can be converted from one water depth to another using the formula:

Table 7 shows the result of a test during which a bubble aerator (2.3 mm long 25 mm diameter perforated plastic pipe with 1.5 mm diameter holes in two lines with a distance of 20 mm between holes) was utilized in a tank with a surface area of 7.5 × 17.5 m.

The scheme of the NOKIA fine bubble aerator is shown in Figure 14. The air intake part of these aerators is made of porous polyethylene material, in pipe (HKP 600) or in disc (HKL 210) form.

They can be dimensioned using the curves given in Figures 15 and 16.

The Flygt 763 type fine bubble aerator is shown in Figure 17. The equation below shows how much oxygen can be dissolved from 1 m air when the temperature of the water is 10°C, its initial oxygen content is zero and when the aerator is placed 1 m below the water surface:

if the value of Q lies between 6 m3h-1 and 30 m3h-1

where

Q = the amount of air flowing through the aerator (the amount of air is calculated for normal conditions, 0°C and 101325 Pa) (m3h-1)

The relation between the air flow and the required pressure is shown in Figure 17.

The cross-section of an ejector for fish pond aeration (Flygt type 4803, 4804) is shown in Figure 18. The primary water flow (1) passes through a Venturi inlet (2) where its velocity increases while its pressure decreases. The low pressure suction chamber (3) is connected to the atmospheric air by a pipe (4) through which air enters the chamber. In the mixing pipe (5) the air and the primary water are mixed together. As the air-water mixture passes through the ejector its velocity decreases in the diffusor pipe while its pressure rises to the pressure at the end of the pipe in the outside water. As an example, graphs are given in Figure 18 that can be used for the dimensioning of Flygt type 4803 and 4804 ejectors.

The basic technical data of two paddle wheel type aerators are given below. These aerators were tested in a concrete tank with 7.5 × 17.5 m surface area. The aerator has a horizontal shaft with two paddle wheels on each end. The shaft is driven by an electric motor and chain. The device is mounted on floats.